Electronic screening for image reproduction is well known in the art. According to a well known technique described in U.S. Pat. No. 4,456,924 of the present assignee, for each screened dot, a multiplicity of coordinates of a laser plotter are translated into screen-cell coordinates. A corresponding cell memory is preloaded by threshold values, defining a cell memory matrix. Input digitized scanned density values of an image, such as a color separation, are compared with the threshold values, cell by cell. The results provide an on/off control input for a laser plotter.
U.S. Pat. No. 4,635,131 describes a method of and apparatus for producing halftone dot film of graduated density distribution. In an electronic image reproduction system, a halftone dot film of a density distribution of a specific pattern is produced by obtaining a value 1 expressed by an equation 1=f(x)+g(y) representative of the specific pattern corresponding to the density value. By superimposing a density signal corresponding to the value 1 on an image signal developed by scanning an original, a reproduction image of the original, modulated by the density signal of a specific pattern, is obtained.
U.S. Pat. No. 4,825,298 to Ikuta and Murai describes a technique for generating a screened reproduction of an image in which the density distribution of a given screen dot is expressed in three dimensions, wherein the area of the screen dot is expressed along X and Y axes and the density is expressed along a Z axis perpendicular thereto. A film coordinate generator generates film coordinates (u,v), corresponding to the position of an exposure beam on a recording film which position is detected by encoders. The film coordinates are in turn supplied to a screen coordinate generator to be converted into virtual screen coordinates (x,y). A beam control signal generator receives the coordinates (x,y) and an image signal corresponding to the position of the exposure beam to output a beam control signal indicating lighting of the exposure beam when one of the coordinates (x,y) is between upper and lower limit values, corresponding to the same, which are previously determined for each combination of the other of the coordinates (x,y) and the density value of the image signal.
U.K. Published Patent Application 2,157,119A to Ikuta describes apparatus which operates similarly to the technique of U.S. Pat. No. 4,456,924 but does not employ a matrix memory. Instead, the threshold function is calculated on the fly in real time or near real time. This apparatus is limited to relatively simple dot configurations.
U.S. Pat. No. 4,556,918 describes a method and apparatus for generating screened halftone images which includes means for assuming an area of halftone dots with desired periodicity and tone reproducibility, subdividing the area into minute cells and setting address values for each of the minute cells, computing out a threshold value of density for each of the cells as a function of the relevant address values and using the computed value as a threshold value of density for the cell, apparatus for obtaining a density-related video signal of the portion of the original corresponding to each of the cells by scanning the original, and apparatus for producing halftone dot signals by comparing the video signals and the threshold value of density with each other.
Applicant/assignee's earlier U.S. Pat. No. 5,079,721 describes apparatus and a technique for generating a screened reproduction of an image including the steps of providing a representation of an original having input density values representing the grey levels of various locations of the original for a given color separation, defining a desired screen dot arrangement for the image, writing screen dots in a line by line fashion, wherein each screen dot is made up of a plurality of lines whose length and location determines the dot configuration and whose length and location is determined by an analog operation employing the input density values of the original and the desired screen dot arrangement.